This application is directed to making bricks and more particularly to a process of manufacturing bricks, and the resultant bricks manufactured by the process, having a high coal ash content. For purposes of the present application, coal ash generally includes both fly ash and bottom ash, and thus reference to any of coal ash, fly ash, and bottom ash can be used interchangeably unless specifically noted otherwise. Selected aspects of the present disclosure may find application in related manufacturing processes.
Brick manufacturing is well known and has been developed over thousands of years so that even today the commercial process is substantially unchanged over many years. The most widely used processes are either to mold or press mold the bricks or alternatively to extrude the bricks. Shale (dry clay) or clay has water added in a desired amount along with other constituents to form a green body. The green body is subsequently dried to remove moisture in the brick, then fired to incipiently fuse or vitrify the components, and next cooled in a controlled manner to result in the final brick that has structural strength and integrity as widely used in the construction industry.
As an example only, in order to make press molded brick the clay or shale is first ground and mixed with water to the desired consistency. The clay is then pressed into molds with a press, and the molded clay is then fired or burned at approximately 900-1150° C. to achieve strength. Alternatively, the bricks subsequently move slowly through a tunnel kiln on conveyors, rails, or kiln cars. The bricks often have added lime, ash, and organic matter to speed the burning process.
In contrast, with extruded brick, ground clay or shale is mixed with 10-25% water and the material is pushed forced through a die to form the desired width and depth. The extrudate is subsequently cut into bricks of the desired length. The cut bricks are then dried for about 20 to 40 hours at approximately 400° F. to harden before being fired in the kiln. Oftentimes, the heat for drying is residual heat from the kiln.
Like any process, manufacturers are constantly evaluating new materials and processes in an effort to reduce the cost of the final product, and particularly without any loss in performance of the manufactured product. Various attempts to incorporate different low cost materials into the brick have been made. For example, pulverized coal-fired power plants generate large quantities of coal ash which includes what is generally referred to as bottom ash and fly ash. Fly ash is the fine-grained, powdery particulate material that is carried off in effluent gas emitted by the power plant. Electrostatic precipitators, filters, cyclones, or other devices are used by the power industry to collect the fly ash and significantly reduce the amount of fly ash released into the atmosphere. Disposal of these residual ash materials is estimated to cost on the order of $85 billion in the US and, if placed in a landfill, the bottom/fly or coal ash is treated like a hazardous material.
Depending upon the coal used as the fuel source, the components of coal ash will in turn vary, but generally speaking all coal ash includes substantial amounts of silicon dioxide (SiO2) and calcium oxide (CaO) that is classified by ASTM standard C618 as either Class C or Class F coal ash. Class F coal ash is a byproduct of bituminous coal with small amounts of CaO. More particular details of Class F coal ash are specified by the chemical composition and physical requirements of ASTM C618. On the other hand, coal ash that is produced from the burning of lignite or sub-bituminous coal has a content of approximately 15-30% CaO and is identified as Class C coal ash. Most Class C coal ashes have self-cementing properties.
To date, incorporating coal ash as a constituent material of brick has met with only limited success. These efforts are heretofore limited by the percentage content of coal ash used in the brick. For example, U.S. Pat. Nos. 4,120,735; 5,358,760; and 7,264,673 are illustrative of prior attempts to incorporate coal ash into bricks.
A need exists to include a greater content of coal ash into bricks. Likewise, incorporating greater coal ash content that improves processing times, cost of manufacture, energy utilization, and maintenance of desired brick quality and strength characteristics, is advantageous from manufacturing, sustainability, and environmental points of view.